Self-thinning of Scots pine across Europe changes with solar radiation, precipitation and temperature but does not show trends in time

Abstract

The effects of site and climate on the self-thinning line, a key characteristic that defines forest dynamics, have been the subject of research for decades. However, contrasting results have generally been found. To adapt management practices for widely distributed species, especially considering the impact of climate change, it is crucial to understand the variables and their effects on the self-thinning line. We conducted a systematic analysis based on 77 trial plots from 62 long-term experiments across Europe, covering the distribution range of Scots pine. Our focus was on unthinned conditions since 1975. Using a linear mixed model, we examined the effects of each statistically significant variable, separating the influences on the slope and the intercept. Our observations revealed that parameters enhancing species tolerance, such as shortwave solar radiation, flatten the slope of the self-thinning line. Conversely, temperature and precipitation, which reduce self-tolerance and increase intraspecific competition, lead to an increase in the slope. Balancing the effects between these aspects results in a maximum negative slope at mid-latitudes. In terms of the intercept, we found compensating effects among the analyzed factors, indicating a monotonic increase with decreasing latitude and increasing radiation. Although there were no significant changes in the self-thinning line since the 1990 s, we observed an increase in mortality, suggesting an accelerated self-thinning process. Site and climatic differences across the distribution range of Scots pine influenced the self-thinning line, whereas no trends with time could be observed. Therefore, management strategies and models based on self-thinning need to be adapted to different latitudes. While climate changes have not yet impacted the trajectory significantly, a continuous rise in temperature, coupled with high precipitation, may accelerate self-thinning and result in increased biomass accumulation.